The present invention relates to an arrangement for controlling a compensated drill floor on a floating drilling or intervention vessel, which is a vessel for carrying out well drilling, completion, workover and/or production operations at the sea. The invention acts to neutralise tidal influence on the position of the drill floor in relation to the sea bottom.
Floating drilling and intervention vessels must have some means to compensate for the heave exerted on the vessel by the sea.
Large floating drilling rigs traditionally include a telescopic coupling device in the riser/drill string. The coupling devices are placed below the drill floor, which means that the drill crew can work almost without regard to the continuously changing influence of the sea. However, the maintenance of this system is both complicated and dangerous.
Many drilling vessels have installed a system where the complete drill floor with derrick is heave-compensated. The floor is mounted on hydraulic cylinders that lift or lower the floor as needed. An example of such a system is disclosed in NO-patent 171958.
Current arrangements for drilling and well service from floating appliances have no arrangements which neutralise the influence of the tide on the drilling system. According to current techniques, all drilling and well service systems are compensated in the compensated draw works in the drilling device. The tidal effect is then adjusted by changing the position of the drill hook. Thus this operation has evident limitations with regard to the safety of the personnel and the lifetime of the coiling tubes, and it makes slim hole drilling from floating appliances difficult.
Presently there is a means of controlling a heave-compensator system by means of an active hydraulic system and accelerometer instruments. This means that the system must operate with error margins and tolerances that are unacceptable for slim hole operations and compensated drill floors.
Further reference to prior art can be found in the applicant""s earlier patents: U.S. Pat. No. 5 727 640, NO 305138 and NO 306026.
An object of the present invention is to indicate an arrangement which can control a compensated drill floor and simultaneously neutralise tidal influence on the drilling systems.
This is achieved in the arrangement according to the present invention which is directed to an arrangement in a floating drilling and intervention vessel, comprising a heave-compensated drill floor suspended in drill floor compensator cylinders. The invention is characterized in that below the compensated drill floor a tidal structure and tidal cylinders are installed, so that by adjusting the tidal cylinders in step with the tide, the drill floor compensator cylinders can cycle with their full stroke regardless of the tide lifting or lowering the vessel in relation to the sea bottom.
It is preferred that below the tidal structure is connected to the compensated drill floor by one or more wires which are fastened to the compensated drill floor in one or more anchoring points. The said one or more wires run down to one or more sheaves installed on the tidal structure and further up to one or more stretch compensators, so that the drill floor compensator cylinders can cycle with their full stroke regardless of the tide lifting or lowering the vessel by controlling said one or more stretch compensators.